I’m idly skimming through Daniel Chandler’s Semiotics for Beginners, in which he says:
Semiotics involves the study not only of what we refer to as “signs” in everyday speech, but of anything which “stands for” something else. In a semiotic sense, signs take the form of words, images, sounds, gestures and objects.
(Oddly, Challis Hodge’s Semiotics: A Primer for Designers seems to be just copied and pasted from this book.)